Modern spark ignition engines, in particular direct injection engines, exhibit increased input of fuel into the oil sector of the crank casing. The proportion of this input of fuel will increase further in future because ethanol is increasingly added to the liquid fuel to be used for refueling and is comparatively volatile and can also penetrate seals. At present, it is planned, at least in Germany, to increase the proportion of ethanol in the fuel from the current 5% to 25%.
This input of fuel has an adverse effect on the service life of the engine oil and also impairs the lubrication capability of the engine oil. For this reason, in addition to reducing the input, attempts are made, above all, to remove the fuel from the oil again as quickly as possible. This is done by means of a venting valve in the crank housing, which venting valve permits the fuel vaporized from the hot engine oil to flow directly into the intake section and therefore into the cylinder. This also prevents the vaporized fuel from being output unburnt into the surroundings. In order to increase the corresponding scavenging current, relatively large engines in particular have not only the venting medium but also a ventilation medium which sucks fresh air from the surroundings into the crank chamber. This air flows past the oil sump and then into the intake section. The proportion of fuel in this scavenging current flowing into the intake section will be referred to below as the outgassing of fuel.
High scavenging currents together with a high proportion of, to a certain extent, sudden outgassing of the fuel (depending on the temperature of the engine oil) can lead to errors in the composition of the fuel/air mixture. In the case of extremely large errors, misdiagnoses can occur in the fuel system diagnosis or even engine stallings. In this context, the risk of engine stallings is particularly high if the engine is in the idling mode or if the engine is changing from relatively high rotational speed into the idling mode. Even in the case of a so-called hot start, in which the engine is warmed up, shut down and started again with a warm engine, outgassing of fuel can lead to the engine not being able to be started.